module/role for url-downloader

url-downloader had already a module but it was not in the module/role
pattern. Split that up and add monitoring and firewall while at it. Also
add support for Ubuntu >= 12.04 since the squid package has undergone
some changes. This duplicates a lot of stuff in install server and as
such a deduplication module is probably warranted. RSpec as well as unit
tests are included, the module and the role class are documented and
minimal documentation in the form of a README file is provided

Change-Id: Ia234bdd7109d9419778e87b6dc8d56abc0763fe7